The gene segments encoding immunoglobulins have the novel property of only being functionally expressed after they undergo specific genetic rearrangements in the developing B lymphocyte. In order to understand the complex controls which regulate antibody production it is necessary to examine the mechanisms by which the gene segments are rearranged and the elements which regulate their transcription. The formation of antibody genes requires site specific translocations of DNA, which for mouse Kappa light chains brings one of several hundred variable segments to one of four joining segments located about 3 kb upstream of a constant segment. Both functional and nonfunctional fragments of the rearranged Kappa locus in plasmacytomas, as well as reciprocal fragments which are generated by the rearrangement, will be characterized by DNA sequencing and compared to determine the consequence of Kappa rearrangements and structural requirements for transcription. The chromosomal location of Kappa rearrangements and origins of non Kappa DNA segments which rearrange into the Kappa locus will be determined. A systematic analysis of rearrangement will be followed in a clonally derived lymphoid cell line which rearranges the Kappa locus in culture. The elements which contribute to transcriptional efficiency will be determined by analyzing the transcriptional competence of aberrant rearrangements which represent mutations or deletions of the Kappa transcription unit. Studies will be initiated to identify specific regions of the DNA required for transcription in vivo by transient expression of Kappa genes in an SV40 vector-HeLa cell system. The ultimate goal of transcription studies will be to determine structures which affect interactions of DNA with elements important for transcriptional regulation in cell lines which can be stimulated or suppressed for antibody production.